1. Field of the Related Art
The present invention relates to a magnetoresistance device for use in a magnetic head, a position sensor, rotation sensor or the like, and also to a method of producing such a magnetoresistance device. The present invention also relates to a magnetic head provided with such a magnetoresistance device.
2. Description of the Related Art
Magnetoresistance reading heads (MR heads) are known in the art. They include an AMR (anisotropic magnetoresistance) head utilizing the anisotropic magnetoresistance effect, and a GMR (giant magnetoresistance) head utilizing spin-dependent scattering of conduction electrons. An example of a GMR head is a spin-valve head disclosed in U.S. Pat. No. 5,159,513. This spin-valve head shows a high magnetoresistance effect in response to a low external magnetic field.
FIGS. 17 and 18 are simplified schematic diagrams illustrating the structure of an AMR head.
In the AMR head shown in FIG. 17, an electrically insulating layer 2 and a ferromagnetic layer (AMR material layer) 3 are successively formed on a soft magnetic layer 1, and antiferromagnetic layers 4 are formed on either end of the ferromagnetic layer 3 in such a manner that the antiferromagnetic layers 4 are spaced by an amount corresponding to the track width. Furthermore, an electrically conductive layer 5 is formed on each antiferromagnetic layer 4. On the other hand, the AMR head shown in FIG. 18 comprises: a multilayer structure including a soft magnetic layer 1, an electrically insulating layer 1, and a ferromagnetic layer 3; magnet layers 6 formed at either side of the multilayer structure in such a manner that the multilayer structure is located between the two magnet layers 6; and an electrically conductive layer 5 formed on each magnet layer 6.
To operate AMR heads of the types described above under optimum conditions, it is required to apply two magnetic bias fields to the ferromagnetic layer 3 having the AMR property.
A first magnetic bias field serves to make the ferromagnetic layer 3 change linearly in resistance in response to a magnetic flux from a magnetic medium. The first magnetic bias field is applied in a direction at a right angle with respect to the surface of the magnetic medium (in the Z direction in FIG. 17) and parallel to the film plane of the ferromagnetic layer 3. The first bias magnetic field is generally called a transverse bias field, and is produced by passing a detection current from the electrically conductive layer 5 into the AMR head.
The second magnetic bias field is generally called a longitudinal bias field, and is applied in a direction parallel to both the film plane of the magnetic medium and the ferromagnetic layer 3 (in the X direction in FIG. 17). The longitudinal bias field serves to suppress Barkhausen noise due to formation of a large number of magnetic domains in the ferromagnetic layer 3, thereby obtaining a smooth and low-noise resistance change in response to the magnetic flux from the magnetic medium.
To suppress the Barkhausen noise, it is required to make the ferromagnetic layer 3 into the form of a single domain. To this end, there are two known methods of applying a longitudinal bias field. In a first method, the magnetic head structure shown in FIG. 18 is employed, and leakage of magnetic flux from the magnet layers 6 disposed at either side of the ferromagnetic layer 3 is used. In the second method, the magnetic head structure shown in FIG. 17 is employed, and an exchange anisotropic magnetic field produced at the interfacial boundary between the antiferromagnetic layer 4 and the ferromagnetic layer 3 is used.
A specific example of a magnetoresistance device utilizing the exchange anisotropic coupling of the antiferromagnetic layer is the exchange bias type device shown in FIG. 19. Another example is shown in FIG. 20, which is knows as the spin-valve type device.
The magnetic head shown in FIG. 19 has a structure similar to that shown in FIG. 17, and comprises a lower insulating layer 21, a ferromagnetic layer 22, a non-magnetic layer 23, and a ferromagnetic layer 24 having the magnetoresistance property wherein these layers are formed into a multilayer structure. Furthermore, antiferromagnetic layers 25 and a lead layer 26 are formed in such a manner that they are spaced by an amount corresponding to the track width TW,
In the structure shown in FIG. 19, as a result of the exchange anisotropic coupling at the interfacial boundary between the ferromagnetic layer 24 and the antiferromagnetic layer 25, a longitudinal bias field is applied to the ferromagnetic layer 24 thereby converting a region B shown in FIG. 19 (where the ferromagnetic layer 24 and the antiferromagnetic layer 25 are in contact with each other) into a single domain directed in the X direction. This induces the ferromagnetic layer 24 in a region A with a width corresponding to the track width to be converted into a single domain in the X direction. A steady-state current is passed from the lead layer 26 into the ferromagnetic layer 24 via the antimagnetic layer 25. When the steady-state current is passed through the ferromagnetic layer 24, a longitudinal bias field in the Z direction caused by magnetostatic coupling energy from the ferromagnetic layer 22 is applied to the ferromagnetic layer 24. If a magnetic leakage field from the magnetic medium is applied to the ferromagnetic layer 24 magnetized by the transverse and longitudinal magnetic bias fields, the electric resistance against the steady-state current varies linearly in proportion to the magnitude of the magnetic leakage field. Therefore, it is possible to detect the magnetic leakage field by detecting the change in the electric resistance.
In the structure shown in FIG. 20, a free ferromagnetic layer 28, a non-magnetic and electrically conductive layer 29, and a ferromagnetic layer 24 are successively formed on a lower insulating layer 21 wherein the free ferromagnetic layer 28, the non-magnetic and electrically conductive layer 29, and the ferromagnetic layer 24 make up a magnetoresistance element. Furthermore, an antiferromagnetic layer 25 and an upper insulating layer 27 are successively formed on the ferromagnetic layer 24.
In the structure shown in FIG. 20, a steady-state current in passed through the magnetoresistance element 19. The magnetization of the ferromagnetic layer 24 is fixed into the Z direction due to the exchange anisotropic coupling with the antiferromagnetic layer 25. If a magnetic leakage field from the magnetic medium moving in the Y direction is applied, the magnetization direction of the free ferromagnetic layer 28 varies, and thus the electric resistance of the magnetoresistance element 19 varies. Therefore, it is possible to detect the magnetic leakage field from the magnetic medium by detecting the change in the electric resistance.
The exchange anisotropic magnetic field generally arises from the exchange interaction of magnetic moments at the interfacial boundary between a ferromagnetic layer and an antiferromagnetic layer. FeMn is well known as an antiferromagnetic material which interacts with a ferromagnetic layer such as a NiFe layer and creates an exchange anisotropic magnetic field. However, FeMn is so poor in corrosion resistance that great degradation in the exchange anisotropic magnetic field occurs due to corrosion which occurs during a production process of a magnetic head and also during the operation of the magnetic head. In some cases, the corrosion damages a magnetic medium. It is known that the temperature in the vicinity of the FeMn layer easily rises to about 120.degree. C. during the operation of the magnetic head due to heat generated by the steady-state detection current. The exchange anisotropic magnetic field produced by the FeMn layer is very sensitive to the change in temperature. That is, the exchange anisotropic magnetic field decreases substantially linearly with the increasing temperature, and the exchange anisotropic magnetic field eventually disappears when the temperature reaches about 150.degree. C. (blocking temperature Tb). This makes it difficult to obtain a stable exchange anisotropic magnetic field.
To solve the above problems, the inventors of the present invention have proposed, in Japanese Patent Application No. 7-78022 filed Apr. 3, 1995, a magnetoresistance device provided with a coercive force enhancement layer made up of .alpha.-Fe.sub.2 O.sub.3, having better corrosion resistance and better temperature characteristic than FeMn described above, in which the rotation of magnetization is pinned by the adjacent ferromagnetic layer having an enhanced coercive force. This structure may be employed to obtain a giant magnetoresistance device.
In the magnetoresistance device disclosed in this patent application, two ferromagnetic layers spaced by a non-magnetic layer are formed and a coercive force enhancement layer made up of .alpha.-Fe.sub.2 O.sub.3 is disposed adjacent to one of the two ferromagnetic layers so that the rotation of magnetization of said one of the two ferromagnetic layers thereby forming a pinned ferromagnetic layer. The other ferromagnetic layer serves as a free ferromagnetic layer in which the rotation of magnetization is allowed. When an external magnetic field is applied, rotation occurs in the magnetization of the free ferromagnetic layer in response to the applied external magnetic field, and thus corresponding resistance change occurs. Because .alpha.-Fe.sub.2 O.sub.3 has a blocking temperature much higher than that of FeMn, the magnetic characteristics of the above magnetoresistance device insensitive to the change in temperature.
Although the above magnetoresistance device provided with the coercive force enhancement layer made up of .alpha.-Fe.sub.2 O.sub.3 has advantages described above, it still has some problems to be solved.
In the magnetoresistance device described above, when the layers are formed on a glass substrate in an amorphous form, an R-H curve such as that shown in FIG. 11 is obtained. In this case, although the device shows a large resistance variation ratio, the squareness ratio of the R-H curve is not high enough. Furthermore, the MR variation in a low magnetic field range is not large enough.
Another problem is that the R--H curve shown in FIG. 11 has a narrow plateau region (the top region in the R-H curve) in which the spins of the pinned ferromagnetic layer and the spins of the free ferromagnetic layer become parallel in opposite directions.
Furthermore, in the magnetoresistance device having the above structure, to pin the rotation of magnetization of the pinned ferromagnetic layer, it is required that the thickness of the coercive force enhancement layer of .alpha.-Fe.sub.2 O.sub.3 should be larger than 600 .ANG.. This causes an increase in the total thickness of the magnetoresistance device. In other words, it is difficult to realize a thin magnetoresistance device.